1. Field of the Invention
The present invention relates generally to an electrical support circuit used with a RF receiver to provide a voltage reference signal to a digital data slicer circuit and, more specifically, to a clamping circuit used in a wireless RF remote controlled system adapted to provide an electrical clamping reference to prevent the data slicer from responding to ambient circuit noise and generating false digital data signals.
2. Description of the Related Art
Remote keyless entry (RKE), and remote alarm systems are known and have been available in motor vehicles for some time, but variations on these, as well as completely new wireless systems are constantly emerging. For example, passive RKE systems are now available that unlock and lock the vehicle simply by the driver moving in and out of the local range of the onboard wireless receiver. “Sentry” ignition keys, which may or may not retain the typical teeth of a key, include preprogrammed circuitry that wirelessly interact with the vehicle to identify the correct ignition key for the particular vehicle. Also, wireless tire pressure monitoring systems are available that provide a constant status of pneumatic pressure within each tire from internal mounted sensors.
In these examples of wireless communication and control systems, the operating environment within a motor vehicle presents unique obstacles that must be overcome to provide the desired results reliably and consistently. Most notably, power consumption of the components in a wireless system is an important design consideration. Characteristically, a wireless system employs at least one receiver and at least one transmitter that are remote from each other and operatively interact through a wireless radio frequency (RF) signal to provide some control function of the greater system. The transmitter typically has to operate and generate an output signal only when activated to achieve a desired action, but the receiver must be powered up and awaiting the transmitter's signal during all possible periods of operation. Thus, a receiver that is installed in a motor vehicle may operate the majority of time on battery power alone, only receiving power from the vehicle's charging system when the engine is running. In fact, in certain motor vehicle applications, the RF wireless systems are disabled or unnecessary during the periods of operation when the engine is running.
For example, regardless of how long a remote keyless entry (RKE) equipped motor vehicle is left sitting, the RKE receiver must remain constantly powered in order to receive the control signal to unlock the vehicle when transmitted by the operator. Additionally, the portions of the RKE system that interpret and process the transmitted signal, such as a microprocessor and other support components, must be able to respond whenever a transmitted signal is received. Since the only source of power when the engine of the vehicle is off is the main battery, the power consumption of the RKE receiver and the signal processing components must be designed to be extremely low. This is especially true in light of the various other onboard vehicle systems that must also draw power when the engine is off. If the required current draw from each of the onboard systems is not kept to very low levels, the drain on the vehicle's main battery will quickly discharge it, thereby disabling the vehicle.
To avoid this problem, vehicle manufacturers have placed restrictions on the current draw of wireless and other systems that must by electrically maintained when the vehicle is not running. Thus, the latest wireless systems now employ low power receivers combined with microprocessors that “sleep” to conserve power until activated by an incoming data signal. This approach has been successful to a point. However, with the addition of even more power consuming systems incorporated into future vehicles, manufactures have demanded even stricter current draw restrictions for new wireless systems. These lower current draw requirements cannot be met by present wireless circuit designs. Generally, this is due to the fact that it is difficult to control false microprocessor wake-up events.
Operationally, when a conventional wireless receiver senses an RF signal in the proper frequency range, the receiver demodulates the encoded digital signal and reshapes it into the original digital data stream. The receiver must then “wake” the microprocessor, so that the microprocessor can decode the digital data stream to first determine if it has the proper identification code and to then determine the requested command (i.e., unlock doors, unlock truck, etc). The waking of the microprocessor and its subsequent data processing actions consume a particular amount of battery power that cumulatively adds up. In order to achieve proper functioning of conventional wireless systems, this signal processing operation must be allowed to occur and power must be consumed even if the receiver occasionally responds to transmitters in the same frequency range but belonging to a different vehicle. In these cases, when the microprocessor recognizes that the subject transmitter has a different identification code it will return to its sleep mode. However, one drawback in the design of present wireless systems is that their ability to respond to incoming RF frequencies also produces a large number of false wake-ups. These false wake-ups repeatedly and cumulatively consume power and waste the limited battery resources.
False wake-ups in conventional wireless systems may be caused when the receiver picks up stray RF signals in the receiver's designed frequency range, by stray harmonics of those frequencies, or by some other electromagnetic interference. This is generally referred to as “external noise”. External noise can be filtered by a variety of methods or by merely reducing the sensitivity of the receiver. On the other hand, reducing the sensitivity of the receiver may shorten the receiver's range and may require a higher-powered transmitter. However, the main cause of false microprocessor wake-ups is from “internal noise” that is generated within portions of the receiver circuitry itself Generally speaking, internal circuit noise is defined as any unwanted electrical signals present in the receiver circuitry. Internal noise is manifested by small fluctuating voltages or currents present as a function of the activity of the electronic components. Internal circuit noise can be minimized, but not eliminated. To avoid problems caused by internal circuit noise, receivers are typically designed to have a high “signal-to-noise” ratio, this represents the level of inherent internal circuit noise that can be tolerated with respect to a signal being processed. Thus, in a receiver having a high signal-to-noise ratio, during the normal RF signal processing, the noise is present merely as a very low-level background interference relative to the signal. This strategy works well in a receiver that processes a continuous signal, such as a radio. However, in applications that require the receiver to wait for an occasional transmission, such as a conventional RKE system or other known wireless RF systems, without a constantly transmitted signal to process, the noise level is often distinct and can be interpreted as a valid signal causing the receiver to repeatedly generate false microprocessor wake-ups.
Up to this point, conventional wireless receiver systems have failed to provide sufficient filtering or noise dampening that adequately attenuates or overcomes ambient internal circuit noise. Thus, they have been unable to avoid the false microprocessor wake-ups that consume excessive amounts of battery power. Therefore, there exists a need in the art to improve the receiver circuit design of low power RF wireless systems to prevent excessive amounts of current draw and power consumption caused by false microprocessor wake-ups in systems that must operate on battery power.